A superconducting fault current limiter (SCFCL) is a device that limits fault currents in a power system. The power system may include transmission and distribution networks to deliver power to differing industrial, commercial, and residential loads. A fault current is an abnormal current in an electrical circuit due to a fault such as a short circuit resulting in a short circuit or fault current. A fault current may occur due to any number of events or failures such as severe weather damaging power lines and components, e.g., lighting striking the power system. When faults occur, a large load appears instantaneously. The network, in response, delivers a large amount of current (i.e. fault current) to this load or, in this case, the faults. This surge or fault current condition is undesirable since it may damage the network or equipment connected to the network.
A SCFCL includes a superconductor positioned in a cryogenic tank. The superconductor is in a superconducting state having zero resistance during a steady state condition. To maintain the superconductor in the superconducting state, the superconductor is operated below its critical temperature, critical current density, and critical magnetic field. If any one of these three is exceeded, the superconductor quenches from its superconducting state to a normal state and exhibits a resistance. To maintain the superconductor at a temperature below its critical temperature, a refrigeration system provides a cryogenic cooling fluid to the cryogenic tank housing the superconductor.
FIG. 1 is a circuit diagram of one power system 100 having a SCFCL 106 consistent with the prior art. The power system 100 also includes an AC power generator 102, a shunt 114 in parallel with the SCFCL 106, a circuit breaker 108 which is normally closed, and differing loads 110. Under steady state conditions, the AC power generator 102 provides power to the loads 110. The circuit breaker 108 is closed and load current flows through conductor 103, the SCFCL 106, and conductor 105 to the loads 110. The superconductor of the SCFCL 106 is in a superconducting state exhibiting zero resistance during the steady state condition. A fault condition may occur as illustrated by the inadvertent path to ground at location 112. In response, the AC power generator 102 attempts to deliver a large amount of fault current. The superconductor quenches and exhibits a resistance much larger than the resistance of the shunt 114. Hence, the fault current is commutated into the shunt 114 which limits the fault current to an acceptable level by reducing the peak to peak value of the fault current before the circuit breaker 108 can open (a conventional circuit breaker 108 typically takes 2 to 3 cycles before opening when operating at a conventional 60 Hz).
A drawback of the conventional SCFCL 106 is that the superconductor portion of the SCFCL 106 may degrade over time. Degradation of the superconductor may be due to mechanically induced stress and/or thermally induced stress. Mechanically induced stress may include fatigue or fault stress from an excessive amount of fault conditions. Thermally induced stress may occur due to the SCFCL 106 handling an excessive amount of fault conditions and/or excessive fault current amplitudes. In addition, aging of the SCFCL may contribute to degradation of the superconductor performance. As one or more portions of the superconductor degrade over time, the critical current density that is necessary to quench the superconductor may be lowered. This may lead to inadvertent quenching and/or unreliability of the SCFCL 106.
Accordingly, there is a need in the art for a SCFCL and method of operating a SCFCL that overcomes the above-described inadequacies and shortcomings.